1. Field of the Invention
The present invention relates to a planet gear apparatus usable such as a decelerator for decelerating the rotation of an actuating system constituting, for example, a joint actuating unit of a robot.
2. Description of the Related Art
In general, in a multi-joint robot, actuators for actuating joints of the robot are provided distributively for the respective joints. Each actuator comprises a motor and a decelerator for decelerating the rotational speed of the motor. It is required that this type of decelerator have a small size, a light weight and a large deceleration ratio, as well as a high torque transmission performance with a high torque and a high torsional strength.
FIG. 3 shows a typical conventional planet gear apparatus 100 used as a decelerator which meets the above requirements. Specifically, in the planet gear apparatus 3, an input shaft 101 is driven to rotate a sun gear 103. The torque of the sun gear 103 is transmitted to three planetary gears 107 arranged equidistantly and meshed with both sun gear 103 and stationary ring gear 105. Thus, the planetary gears 107 revolve around the sun gear 103, while rotating about their own axes. Each planetary gear 107 is also meshed with a rotary ring gear 109, in addition to the stationary ring gear 105. The number of teeth of the rotary ring gear 109 is slightly different from that of the stationary ring gear 105 by, for example, two or three. Thus, an output shaft 111 coupled to the rotary ring gear 109 is rotated. Specifically, when each planetary gear 107 revolves around the sun gear 103 while rotating about its own axis, the rotary ring gear 109 rotates in accordance with the difference in the number of teeth between the rotary ring gear 109 and the stationary ring gear 105. As a result, the rotation speed of the output shaft 111 is decelerated relative to that of the input shaft 101.
In the conventional planet gear apparatus 100 having the above structure, cylindrical involute spur gears are generally used as the respective gears. The stationary ring gear 105 and the rotary ring gear 109 are coaxial with the sun gear 103. The rotary shaft of the each planetary gear 107 meshed with the stationary ring gear 105 and rotary ring gear 109 is parallel to the rotary shafts of the stationary ring gear 105, rotary ring gear 109 and sun gear 103. It is necessary that the stationary ring gear 105 and rotary ring gear 109, which are different from each other in the number of teeth and arranged coaxially, be precisely meshed with each planetary gear 107. For this purpose, the addendum modification factor of the ring gear having a less number of teeth, e.g. the stationary ring gear 105, is increased. Accordingly, as shown in FIG. 4, the diameter of an addendum circle 105a of the stationary ring gear 105 is designed to be substantially equal to that of an addendum circle 109a of the rotary ring gear 109. The three planetary gears 107 meshed with the stationary ring gear 105 and rotary ring gear 106 have common specifications and integrated. Thus, the diameters of the addendum circles 107a of the portions of the planetary gears 107, which are meshed with the stationary ring gear 105 and rotary ring gear 109, are equal to one another.
In the planet gear apparatus 100 having the above structure,
1) a deceleration ratio is determined by differential movement corresponding to a difference in the number of teeth between the stationary ring gear 105 and rotary ring gear 109,
2) since the planetary gears 107 meshed with the stationary ring gear 105 and rotary ring gear 109 are integrated into one body, as stated above, the amount of deformation of the planetary gears 107 is small, and
3) since the output from the rotary ring gear 109 can be derived directly, the torsional strength of the apparatus is high and the apparatus has a small number of parts, a small size and a light weight.
The conventional planet gear apparatus having the above structure, however, has the following problem.
There has recently been a demand for high-speed actuation of an industrial robot. For example, a decelerator having a low deceleration ratio, e.g. 30:1 to 80:1, is needed. To meet the need, in a conventional planet gear apparatus, when the number of planet gears is three and the difference in the number of teeth between the rotary ring gear and the stationary ring gear is three, a deceleration ratio of about 60:1 or more can be obtained. However, it is not possible to obtain a deceleration ratio less than this value.
On the other hand, in the conventional planet gear apparatus, for example, if it is possible to use three planetary gears and set the different in number of teeth at "6", the deceleration ratio of the planetary gears can substantially be reduced to 1/2 and the above requirement can be met.
However, in the case of the conventional structure, if an attempt is made to mesh the planetary gears with both ring gears precisely, the addendum modification factor of the ring gear having a less number of teeth becomes high, i.e. three or more. Thus, it is practically impossible to constitute the planet gear apparatus. Because of the above, the conventional planet gear apparatus having a small size, a light weight and a "high deceleration ratio" can be obtained, but it is not possible to achieve a "low deceleration ratio" which can meet the requirement for "high-speed actuation" in modern industrial robots.